The disclosure relates generally to a scanning system, and more particularly concerns to calibration of scanners for the scanning of original hard-copy images into electronic form, as would be found, for example, in a digital copier or other scanner.
Document handlers (which are also known as a “constant-velocity transports,” or CVTs) are devices that draw individual sheets from a stack of sheets, and sequentially allow the image on each sheet to be recorded, typically by a photoreceptor (in a “lightlens” or analog copier) or by a photosensitive device (in a digital copier, scanner, or facsimile). In a common arrangement, a document handler also has a conventional main platen, on which single sheets can be manually placed, as well as a smaller CVT area, typically adjacent the main platen, which is used by the document handler when sheets are being passed therethrough. In a typical design, when a single sheet is being recorded through the main platen, the photosensitive device with associated light source (or “scan head”) is moved relative to the platen to record the entire image; when the document handler is being used to expose images through the smaller CVT area, a photosensitive device is typically left stationary under the smaller CVT area, and the motion of the sheet caused by the document handler provides the necessary relative motion of each sheet past the photosensitive device.
In the practical, day-to-day use of a document handler, the responsivity of the document handler and platen change over time and the changes in the system must be compensated for periodically, in order to ensure consistent output. The periodic compensation of the system for changes in responsivity is known as “calibration” of the scanning device. Common sources of long-term performance variation over time include the declining intensity, primarily due to process variations, of the internal light source. Another performance variation can be attributed to platen and CVT scanning, typically at different heights, due to illumination intensity changes (depth of illumination) at different heights from the light source. A light source targeting platen scanning would require one calibration at the platen height which would be at a different calibration from that required for CVT scanning due to the difference in scan height and the associated depth of illumination change compared to documents at the platen. Current calibration techniques tend to split the difference and balance Image Quality (IQ) between the two scanning modes generating a compromise calibration.
There is, therefore, a need for methods and systems that can optimally control the calibration of a scanner and separately a document handler based upon imaging height, thereby correcting for process variations and other factors that may adversely affect exposure of an image being scanned. This allows the selection of the optimum calibration for a particular system to give the best range and signal characteristics.